The present invention relates to fastener, and, in particular, relates to composite fasteners.
For fastening together portions of certain highly stressed constructions, such as aircraft or other assemblies requiring high strength and light weight, many variations of fasteners have been suggested. Most of these prior art fasteners have been made of a core material to which a different material has been applied as a thin protective coating, as by plating, galvanizing, and other application methods. In these fasteners, the reduction in weight has caused a corresponding reduction in strength and/or the ability to withstand one or more of the other forces caused by the stresses to which the assembly may be subjected.
Attempts to overcome the disadvantages of the prior art fasteners have been successful in part but have introduced other shortcomings. Typically, a molded, non-metallic fastener having an external coating of metal provides a lightweight fastener, but is unlikely to have the desired strength to withstand severe stress and tension forces without a significant increase in size. Also, metal coated non-metallic fasteners typically cannot be used in conjunction with metallic structures since the relatively thin metallic coating can be quickly destroyed by the metallic structures, permitting the assembled structures to act against the more fragile core material. Thus, under the high stress conditions in which these fasteners may be used, failures can occur by cutting through or shearing the core.
Another prior art fastener such as shown in U.S. Pat. No. 4,824,314, places a core material such as epoxy-graphite or alloys into a blank shell and the combination is formed into a bolt by the described technique. The shell would have a thickness such as would allow warm rolling and pressure forming. The core material is homogeneous.
Other prior art fasteners having a totally composite construction use selectively oriented resin-impregnated fibers to achieve strength. These fasteners lack sufficient strength both in tensile and shear loading to be of value in critical areas such as aircraft construction. Further, delamination of the fasteners has been a problem and as such special devices are used to reduce this damage. Examples of these are shown in U.S. Pat. Nos. 2,949,054; 4,623,290; and 4,717,302.
Thus, there is clearly a need for a fastener having a composite matrix construction within a metal shell.